Airbnb built its business partly by sending automated emails to people who placed rental listings on Craigslist, according to entrepreneur Dave Gooden, who works in the vacation rental industry.

Over the weekend, peer-to-peer rentals site Airbnb raised $100 million at a $1 billion valuation. The company is only three years old, but is on track to earn $25 million on $500 million in sales, according to estimates.

Gooden began looking at Airbnb about a year and a half ago, when he noticed its fast growth. He didn't see the company spending a lot of money on advertising, and couldn't believe that word of mouth and tech blogs were responsible for its growth.

Instead, he had a hunch that Airbnb was using Craigslist to find listings, then soliciting the people who posted the Craigslist ads and asking them to place their ads on Airbnb instead.

To test his hunch, he placed a vacation rental on Craigslist. He made sure to check the box saying he did not want commercial solicitations.

Hours later, he had an email from a young lady who said that Airbnb was a great site, and that he should list his rental on it. She included a direct link in case he wanted to check it out for himself.

No problem, right? Just a zealous fan.

Except then Gooden did a more complicated test. He created his own test site for vacation rental listings, then used the same tactics he believed Airbnb was using. He managed to get 1,000 or so vacation listings on this fake site.

Then, he turned around and started re-listing those rentals on Craigslist.

Every time he did it, within 12 hours he got an email from a supposed Airbnb fan. Each time, it said that his listing was "one of the nicest listings in Craigslist in the [x] area" and recommended that he list it on Airbnb.

The emails were almost exactly the same. Only the details -- including the supposed sender and email address -- had changed.